CONTREX CX-1010 - Manual (Page 4)

Since the resolution byte can be predetermined (decimal places), and the definition byte
is not important, you can easily pre-establish the 2 bytes to append to the outgoing
"write" parameters. If you treat these as 1 word, or as a 2 byte integer, the following
values will place the correct resolution byte (decimal places) in the "high" byte of this
word or integer:
decimal places value of 2 byte integer
0
0
1 256
2 512
3 768
4 1024
Explicit Messaging:
While the CX1010 DeviceNet option card is capable of explicit messaging, most users do
not use this feature. There are some initializing activity that requires some explicit
messages, but the scanner card (host) will take care of these automatically. The User
does not usually have to deal with these at all. Just follow the setup procedures for your
scanner.
An explicit message can be sent to change the communications time-out values in the
CX1010 card. This is known as the EPR, or Expected Packet Rate. The default time-out
is 10 sec, but can be set to indefinite. However, the easiest solution is to tell the scanner
to "Ping" the CX1010. The scanner will send a "no-op" message at regular intervals to
keep the node alive if scanning is interrupted for any reason.
Faults and Errors:
An explicit message can query the CX1010 to determine a communication fault, or
reason for a rejected parameter value. Most users do NOT go to that much trouble. The
scanner itself will report many types of communications errors in an easier to use format.
Most users that need to verify that a "write" action was successful will use a write &
read sequence. Just add the write parameter to the read list. Then write the new value,
wait 1 scan, and compare the read value to the original write value.
Auto/Manual Selection:
The rejection of a write value is not always an error. This feature can be used to toggle
between an Auto Mode (PLC determines setpoint), and a Manual Mode (CX1010
operator manually enters a setpoint). Of course the operator can use other means to
acquire control over the setpoint (i.e. direct the PLC scan to a block parameter and
change blocks via a switch), but there also is a way for the PLC to make that decision and
override the operator's setpoint if needed. The PLC would send a "reasonable" setpoint
value any time it wanted to take control. Since polling happens very rapidly, any setpoint
entered by the operator would be immediately over-written by the value from the PLC.